I did a bottom job on my 18 earlier this year. The wear was bad enough that I layed down some glass in the bad areas first. Then filled in with putty, and then painted on gelcoat.
If your boat isn't worn bad enough that you need glass, then I think there's no problem going right to the gelcoat. If you have any deep scratches, you probably want to fill them with putty first.
Anyway, for my gelcoat application, I used a brush to paint it on (cheap 1" wide brush from harbor freight, you can get a box of like 3 dozen of them for $5). Pull out all the loose bristles first before using so they don't end up in your work. I got the gelcoat from west marine, it had the wax already in it, so you just mix it up with the catelyst and paint it on.
I did about three coats of gelcoat, sanding between coats. The first coat was mixed with filler (microfibers) to make a thick paste, then straight gelcoat on the next coat and on the last coat, I cut it with acetone to make it thin to minimize finish sanding.
Thanks much, I'll give it a shot. I'm curious as to why you used such a small (1") brush? Is it because the gelcoat is considerably thicker than paint? Is the "roll and tip" method out of the question with acetone-thinned gelcoat? Also, if I mark off the work area with painters tape will I be able to peel that off when I'm finished (or do I need to re-tape for each coat)?